spring boot 枚举使用的坑

Posted 2022-09-08 hankuikui

java 枚举的功能挺多，但是坑更多，使用的时候要注意。如下面这个枚举。

@Getter
@AllArgsConstructor
public enum  EnumExpenseType implements BaseEnum 
    小欢喜(1),
    大欢喜(2);

    private final int value;

咋一看，没什么问题，但是具体使用过程中，总是会出问题。原因就是这个枚举没有按照从0开始索引，除此之外即使从0开始，中间有断的索引也会有问题。主要出现在以下方面：

1. 在controller的方法中，比如以这个枚举为参数，如下代码：

    @RequestMapping("/**")
    public String getRejectReasons(EnumExpenseType type) 
        return "";
    

前台传入的参数如果是type:1, 那它值应该是：小欢喜，实际上呢？

Caused by: java.lang.IllegalArgumentException: No enum constant com.**.EnumReasonType.1
at java.lang.Enum.valueOf(Enum.java:238) ~[?:1.8.0_111]
at org.springframework.core.convert.support.StringToEnumConverterFactory$StringToEnum.convert(StringToEnumConverterFactory.java:52) ~[spring-core-5.1.8.RELEASE.jar:5.1.8.RELEASE]
at org.springframework.core.convert.support.StringToEnumConverterFactory$StringToEnum.convert(StringToEnumConverterFactory.java:38) ~[spring-core-5.1.8.RELEASE.jar:5.1.8.RELEASE]
at org.springframework.core.convert.support.GenericConversionService$ConverterFactoryAdapter.convert(GenericConversionService.java:436) ~[spring-core-5.1.8.RELEASE.jar:5.1.8.RELEASE]
at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:41) ~[spring-core-5.1.8.RELEASE.jar:5.1.8.RELEASE]
at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:191) ~[spring-core-5.1.8.RELEASE.jar:5.1.8.RELEASE]
at org.springframework.beans.TypeConverterDelegate.convertIfNecessary(TypeConverterDelegate.java:129) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:73) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:53) ~[spring-beans-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.validation.DataBinder.convertIfNecessary(DataBinder.java:693) ~[spring-context-5.1.7.RELEASE.jar:5.1.7.RELEASE]
at org.springframework.web.method.annotation.AbstractNamedValueMethodArgumentResolver.resolveArgument(AbstractNamedValueMethodArgumentResolver.java:124) ~[spring-web-5.1.7.RELEASE.jar:5.1.7.RELEASE]
... 81 more

Failed to convert value of type ‘java.lang.String‘ to required type ‘com.**.EnumExpenseType‘; 
nested exception is org.springframework.core.convert.ConversionFailedException: 
Failed to convert from type [java.lang.String] to type [com.**.EnumExpenseType] for value ‘1‘; 
nested exception is java.lang.IllegalArgumentException: No enum constant com.***.EnumExpenseType.1
实际上它却报了个错。转换失败了。
查看报错信息，可以定位到是spring框架中StringToEnumConverterFactory中转换失败，具体代码如下：

private static class StringToEnum<T extends Enum> implements Converter<String, T> 

        private final Class<T> enumType;

        public StringToEnum(Class<T> enumType) 
            this.enumType = enumType;
        

        @Override
        public T convert(String source) 
            if (source.isEmpty()) 
                // It‘s an empty enum identifier: reset the enum value to null.
                return null;
            
            return (T) Enum.valueOf(this.enumType, source.trim()); 
        
    

是Enum.valueOf这里报错，Enum.valueOf的后面的值并不是我们的value,而是name(这里的小欢喜)。

所以，我们不能使用这个spring提供converter，需要自定义一个：StringToEnumConverterFactory 

public class StringToEnumConverterFactory implements ConverterFactory<String, BaseEnum> 

    private static final Map<Class, Converter> converterMap =  new HashMap<>();

    @Override
    public <T extends BaseEnum> Converter<String, T> getConverter(Class<T> targetType) 
        Converter<String, T> converter = converterMap.get(targetType);
        if(converter == null) 
            converter = new StringToEnumConverter<>(targetType);
            converterMap.put(targetType, converter);
        
        return converter;
    

    class StringToEnumConverter<T extends BaseEnum> implements Converter<String, T> 
        private Map<String, T> enumMap = new HashMap<>();

        StringToEnumConverter(Class<T> enumType) 
            T[] enums = enumType.getEnumConstants();
            for(T e : enums) 
                enumMap.put(String.valueOf(e.getValue()), e);
            
        

        @Override
        public T convert(String source) 

            T t = enumMap.get(source);
            if (t == null) 
                // 异常可以稍后去捕获
                throw new IllegalArgumentException("No element matches " + source);
            
            return t;
        
    

然后再将这个工厂配置到项目中WebMvcConfigurationSupport：

@Override
    public void addFormatters(FormatterRegistry registry) 
        registry.addConverterFactory(new StringToEnumConverterFactory());

    

意思就是string 转 BaesEnum都走这个converter。

至此这个坑就算解决了。

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